This invention relates to a developing device for an electrophotographic copying machine, and more particularly to a developing device which is characterized by the feature that the surface of the parts located within the developing device which will come into contact with a developer material during machine operation is coated with a conductive material having no affinity with the developer material.
Generally, an electrophotographic copying machine includes a light-sensitive member having a photoconductive surface layer, on which an electrostatic charge pattern is formed by uniformly charging the surface followed by image exposure so as to discharge the charge in proportion to the intensity of the exposure light. The electrostatic charge pattern thus formed is then developed with a developer to obtain a visible powder image which is in turn transferred onto a support such as copying paper and fixed thereon to obtain a permanent image. In an electrophotographic copying machine of the type described herein, there is employed a developing material consisting of finely divided pigmented resinous particles (toner) and relatively coarse granules (carrier) which attract each other as a result of triboelectrification. Carrier is generally prepared by coating granules of core material such as sand, glass and steel with a suitable coating material which, when subjected to triboelectrification, gives to the carrier a certain charge of the polarity distant from that of the toner. When the toner and carrier are mixed and undergo frictional contact, charges of opposite polarities are induced on the toner and carrier, respectively, causing them to adhere to each other.
One method in which a mixture of toner and carrier is used as a developer is known as cascade development. This method includes cascading the above mentioned developer over the surface of a light-sensitive plate so as to allow the toner to adhere to the image areas of the latent image on the light-sensitive plate. Cascade development has been widely used in the art of electrophotography because it is highly effective and can be carried out at low cost. One disadvantage of cascade development, however, is that due to the charge and the finely divided particulate nature of the developer, the toner tends to adhere to various parts such as the developing electrode within the developing device and set or solidify thereon. The toner rigidly adhering to the developing electrode will form an insulating layer on the surface thereof thereby causing the degradation of the effectiveness of the developing electrode. In addition, the developer will be charged at an extremely high potential since the charge thereof does not leak through the developing device, thus resulting in the lowering of the density of developed image.